Thermoplastic polyurethane that exhibits a low formation of combustion gas when burned

ABSTRACT

The invention relates to flame resistant thermoplastic polyurethane which exhibits a low formation of combustion gas when burned and contains a sufficient quantity of a flame proofing agent to achieve the rating V2, V1 or V0 according to Vertical Burning Test UL94 V of Underwriters Laboratories for thermoplastic polyurethane.

The invention relates to flame-retardant thermoplastic polyurethanewhich generates little smoke during combustion and which comprises flameretardant in an amount which achieves the V-2, V-1, or V-0classification for the thermoplastic polyurethane in the UnderwritersLaboratories UL 94 V vertical burning test.

Thermoplastic polyurethanes (hereinafter termed TPUs) aresemicrystalline materials and belong to the thermoplastic elastomersclass. They have good combinatorial properties, such as low abrasion andgood chemicals resistance. TPU is generally combustible, and flameretardants therefore need to be added if flame retardancy is to beachieved. Use is usually made of halogen-containing compounds incombination with antimony derivatives. Use is also increasingly beingmade of halogen-free flame retardants, in particular those based onnitrogen compounds and on phosphorus compounds.

EP-A-617 079 describes the use of melamine derivatives in combinationwith organic phosphates or phosphonates for preparing flame-retardantTPU. Although the TPUs disclosed as preferred in the publication areadvantageous for flame retardancy, the combustion of these substancesgenerates undesirable smoke. Particularly following Europeanharmonization of the relevant test specifications, it is desirable toreduce smoke generation when TPU undergoes combustion.

It is an object of the present invention, therefore, to providethermoplastic polyurethanes which are flame-retardant and also generatelittle smoke during combustion, and therefore comply with the relevanttest specifications.

We have found that this object is achieved by way of thermoplasticpolyurethanes which firstly comprise flame retardant and secondlycomprise no, or only small amounts of, aromatic hydrocarbon groups.Surprisingly, it has been found that a high content of aromaticcompounds in the polyurethanes or in the additives leads to severe smokegeneration.

The invention therefore provides a flame-retardant thermoplasticpolyurethane prepared by reacting

a) polyisocyanates with

b) compounds having at least two hydrogen atoms reactive towardisocyanate, and

c) chain extenders,

d) flame retardant,

e) catalysts where appropriate, and

f) additives where appropriate,

the amount of flame retardant (d) used being such as to achieve theV-2., V-1, or V-0 classification in the Underwriters Laboratories UL 94vertical burning test, and the content of compounds having aromatichydrocarbon groups in components b) to f) being below 5% by weight,based on the total weight of the thermoplastic polyurethane, and aprocess for its preparation.

The invention also provides the use of the thermoplastic polyurethanesof the invention for preparing thermoplastic polyurethanes whichgenerate little smoke during combustion and which, in accordance withthe Indice de fumée of NF-F16-101, have a maximum smoke density (Dm) of300, and have a maximum degree of smoke-darkening after 4 minutes(VOF-4) of 650.

For the purposes of this application, aromatic hydrocarbon groups arecyclic hydrocarbon compounds and, respectively, cyclic hydrocarbonstructure fragments of compounds which have a conjugated π-electronsystem with (4n+2) π-electrons, n being a natural number. n ispreferably 1. The rings here can therefore be isolated or condensedrings.

Examples of compounds which contain these aromatic hydrocarbon groupsare benzene, toluene, and triphenyl phosphate. The term “aromatichydrocarbon groups” does not apply to heteroaromatics, such as furan,thiophen, melamine or pyridine.

Components (b) to (f) of the thermoplastic polyurethanes of theinvention have, based on the total weight of the thermoplasticpolyurethanes, a content of below 5% by weight of compounds whichcontain aromatic hydrocarbon groups. Components (b) to (f) of thethermoplastic polyurethanes of the invention preferably have a contentbelow 2% by weight, more preferably below 1% by weight, particularlypreferably below 0.1% by weight, based on the total weight of thethermoplastic polyurethanes, of compounds which contain aromatichydrocarbon groups. In particular, it is preferable for no components(b) to (f) which contain aromatic hydrocarbon groups to be used toprepare the thermoplastic polyurethanes of the invention.

The following description applies to the TPUs of the invention and tothe components (a) to (f) of the composition:

a) organic polyisocyanates which may be used are aliphatic,cycloaliphatic, or aromatic polyisocyanates, preferably diisocyanates.Specific examples which may be mentioned are:

aliphatic diisocyanates, such as hexamethylene 1,6-diisocyanate,2-methylpentamethylene 1,5-diisocyanate, cycloaliphatic diisocyanates,such as isophorone diisocyanate or cyclohexane 1,4-diisocyanate,aromatic diisocyanates, such as tolylene 2,4- or 2,6-diisocyanate,diphenylmethane 4,4′-, 2,4′-, and 2,2′-diisocyanate, H12-MDI, oradvantageously substantially pure diphenylmethane 4,4′-diisocyanate.

In one preferred embodiment, use is made of polyisocyanates withoutaromatic groups, i.e. preference is given to the use of aliphatic orcycloaliphatic diisocyanates. It is particularly preferable to usehexamethylene diisocyanate.

In one preferred embodiment, therefore, components (a) to (f) of thethermoplastic polyurethanes of the invention have, based on the totalweight of the thermoplastic polyurethanes, a content of below 5% byweight, more preferably below 2% by weight, particularly preferablybelow 1% by weight, in particular below 0.1% by weight, of compoundswhich contain aromatic hydrocarbon groups. In one very preferredembodiment, none of the components a) to f) contains aromatichydrocarbon groups.

Where appropriate, subordinate amounts, e.g. amounts of up to 3% byweight, based on the organic diisocyanate, of trifunctionalpolyisocyanate or of polyisocyanate of higher functionality may replacethe diisocyanates whose use is preferred, but the amount of thepolyisocyanate should be restricted so that the polyurethanes obtainedremain thermoplastically processable. Any relatively large amount ofthese isocyanates which are more than bifunctional is advantageouslycompensated by concomitant use of less-than-bifunctional compoundshaving reactive hydrogen atoms, so as to avoid any excessive chemicalcrosslinking of the polyurethane.

b) The compounds used having at least two hydrogen atoms reactive towardisocyanate generally comprise relatively high-molecular-weightpolyhydroxy compounds with molecular weights of from 500 to 8 000.Examples of those suitable are polyether polyols and polyester polyols,preferably polyetherdiols and polyesterdiols. An example of a compoundused is polybutadienediol, which also achieves good results in thepreparation of crosslinkable TPUs.. Other compounds which may be usedare other polymers containing hydroxy groups and having ether or estergroups in the polymer chain, for example polyacetals, such aspolyoxymethylenes, and especially water-insoluble formals, e.g.polybutanediol formal and polyhexanediol formal, and polycarbonates, inparticular those made from diphenyl carbonate and 1,6-hexanediol, bytransesterification. The polyhydroxy compounds should be at leastpredominantly linear and have to have a substantially bifunctionalstructure for the purposes of the isocyanate reaction. The polyhydroxycompounds mentioned may be used as single components or in the form ofmixtures.

Suitable polyether polyols may be prepared by known processes, forexample by anionic polymerization of alkylene oxides using, for example,alkali metal hydroxides, such as sodium hydroxide or potassiumhydroxide, or alkali metal alkoxides as catalysts, and with addition ofat least one starter molecule which contains from 2 to 3, preferably 2,reactive hydrogen atoms, or by cationic polymerization, using Lewisacids as catalysts, starting from one or more alkylene oxides havingfrom 2 to 4 carbon atoms in the alkylene radical.

Other polyetherols which may be used are those which may be calledlow-unsaturation polyetherols. For the purposes of this invention,low-unsaturation polyols are in particular polyether alcohols whosecontent of unsaturated compounds is smaller than 0.02 meq/g, preferablysmaller than 0.01 meq/g. These polyether alcohols are mostly preparedvia addition reactions of alkylene oxides, in particular ethylene oxide,propylene oxide, or a mixture of these, onto the starters describedabove, in the presence of highly active catalysts. Examples of thesehighly active catalysts are cesium hydroxide or multimetal cyanidecatalysts, preferably double-metal cyanide catalysts, also termed DMCcatalysts. A DMC catalyst often used is zinc hexacyanocobaltate.

Examples of suitable alkylene oxides are tetrahydrofuran, propylene1,3-oxide, butylene 1,2- or 2,3-oxide, and particularly preferablyethylene oxide and propylene 1,2-oxide. The alkylene oxides may be usedindividually, in succession one after the other, or as mixtures.Examples of starter molecules which may be used are: water, organicdicarboxylic acids, such as succinic acid, adipic acid, and/or glutaricacid, alkanolamines, e.g.. ethanolamine, N-alkylalkanolamines,N-alkyldialkanolamines, e.g. N-methyl- and N-ethyldiethanolamine, andpreferably bifunctional alcohols, e.g. ethanediol, 1,2-propanediol,1,3-propanediol, 1,4-butanediol, or diethylene glycol, where these may,where appropriate, contain ether bridges. The starting molecules may beused alone or as a mixture.

It is preferable to use polyetherols based on propylene 1,2-oxide andethylene oxide, where more than 50%, preferably from 60 to 80%, of theOH groups in these are primary hydroxy groups, and where these have atleast a portion of the ethylene oxide arranged as a terminal block. Thepolymerization products of tetrahydrofuran which contain hydroxy groupsare also particularly preferably suitable.

The substantially linear polyetherols usually have number-average molarmasses of from 500 to 8 000 g/mol, preferably from 600 to 6 000 g/mol,and in particular from 800 to 3 500 g/mol, and the polyoxytetramethyleneglycols here preferably have molar masses of from 500 to 2 800. They maybe used either individually or else in the form of a mixture with oneanother.

Suitable polyester polyols, preferably polyester diols, may be preparedfrom dicarboxylic acids having from 2 to 12, preferably from 4 to 6,carbon atoms and diols, for example. Examples of dicarboxylic acidswhich may be used are: aliphatic dicarboxylic acids, such as succinicacid, glutaric acid, adipic acid, suberic acid, azelaic acid, andsebacic acid, and aromatic dicarboxylic acids, such as phthalic acid,isophthalic acid, and terephthalic acid. The dicarboxylic acids may beused individually or as a mixture, e.g. in the form of a succinic,glutaric, and adipic acid mixture. To prepare the polyesterols, it can,where appropriate, be advantageous to use the appropriate dicarboxylicderivatives instead of the dicarboxylic acids, for example a mono- ordiester of a dicarboxylic acid having from 1 to 4 carbon atoms in thealcohol radical, dicarboxylic anhydrides, or dicarboxylic dichlorides.Examples of the diols are glycols having from 2 to 10 carbon atoms,preferably from 2 to 6 carbon atoms, for example ethylene glycol,diethylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol,1,10-decanediol, 2,2-dimethyl-1,3-propanediol, 1,3-propanediol, anddipropylene glycol. Depending on the properties desired, the diols maybe used alone or, where appropriate, in a mixture with one another.

Esters of carbonic acid with the diols mentioned, in particular withthose having from 4 to 6 carbon atoms, such as 1,4-butanediol and/or1,6-hexanediol, are also suitable. Condensation products ofω-hydroxycarboxylic acids, such as ω-hydroxycaproic acid, and preferablypolymerization products of lactones, such as unsubstituted orsubstituted ω-caprolactone, are also suitable.

Preferred polyester diols used are ethanediol polyadipates,1,4-butanediol polyadipates, ethanediol 1,4-butanediol polyadipates,1,6-hexanediol neopentyl glycol polyadipates, 1,6-hexanediol1,4-butanediol polyadipates, and polycaprolactones.

The number-average molar masses of the polyesterdiols are generally from500 to 6 000 g/mol, preferably from 800 to 3 500 g/mol.

In one preferred embodiment, the component (b) used comprises apolyesterol based on an alkylene glycol adipate, the alkylene radicalhaving from 2 to 6 carbon atoms. The use of alkylene glycol adipates, inparticular having an alkylene group having from 2 to 6 carbon atoms,provides the possibility of using only small amounts of, or preferablyno, flame retardant (d) to achieve the desired flame retardancy (V-2,V-1 classification in the Underwriters Laboratories UL 94 verticalburning test).

c) The chain extenders used with molecular weights which are generallyfrom 60 to 399, preferably from 65 to 300, are preferably aliphaticdiols having from 2 to 12 carbon atoms, preferably having 2, 4, or 6carbon atoms, e.g. ethanediol, 1,6-hexanediol, diethylene glycol,dipropylene glycol, and in particular 1,4-butanediol. However, diestersof terephthalic acid with glycols having from 2 to 4 carbon atoms arealso suitable, as are polytetramethylene glycols with molecular weightsof from 162 to 378.

The molar ratios of the components (b) and (c) of the composition may bevaried relatively widely to adjust the hardness of the TPUs. Success hasbeen obtained when the molar ratio of component (b) to all of the chainextenders (c) to be used is from 10:1 to 1:10, in particular from 1:1 to1:4, the hardness of the TPUs rising as the content of (c) increases.

d) The component (d) used in the present invention may comprisehalogen-containing or preferably halogen-free flame retardants.. Theselection of the flame retardant used is such that components b) to f)have a content of below 5% by weight of compounds having aromatichydrocarbon groups, based on the total weight of components b) to f).The use of flame retardants which have no aromatic hydrocarbon groups istherefore preferred.

The halogen-containing flame retardants used may comprise a wide varietyof fluorinated or preferably chlorinated or brominated compounds. Anexample of an effective flame retardant is chlorinated polyethylene,where appropriate with antimony (III) oxide as synergist and/or zincborate. To improve flame retardancy, a number of other metal oxides mayalso be added to the TPU, examples being ZnO, B₂O₃, Fe₂O₃, CaO.Polytetrafluoroethylene and silica in very small proportions is asuitable antidrip agent.

Other suitable halogen-free flame retardants, alongside aluminatrihydrate, for example, and magnesium hydroxide for particularlylow-melting TPUs, are the triesters of phosphoric acid, for exampletrialkyl phosphates. Particular preference is given to oligomericphosphoric esters or, respectively, phosphonic esters, and also tocyclic phosphates which derive from pentaerythritol or from neopentylglycol. These phosphoric esters may be used alone or in a mixture withone another, or in a mixture with phosphonic esters. However, it isusual to use phosphoric esters or phosphonic esters.

In one particularly suitable flame retardant combination, the phosphoricesters and/or phosphonic esters are used for the TPU in mixturestogether with one or more melamine derivatives. The ratio by weight ofphosphate and phosphonate to melamine derivative here is then preferablyin the range from 5:1 to 1:5. Melamine derivatives used here preferablycomprise melamine cyanurate, melamine phosphate, melamine borate,particularly preferably melamine cyanurate.

In one particularly preferred embodiment, melamine derivatives are usedas flame retardant (d) without addition of phosphoric esters. Inparticular, melamine cyanurate is used as sole flame retardant (d).

The amount added of the flame retardants is sufficient to make thethermoplastic polyurethane of the invention sufficiently flame-retardantto achieve the V-2, V-1, or V-0 classification in the UnderwritersLaboratories UL 94 vertical burning test. The amount of flame retardantis preferably sufficient to achieve the classification V-1 or V-0,particularly preferably V-0, to UL 94.

The amount needed of-flame retardant depends on the components used (a)to (c), and (e) and (f). It is usual and general for the amount of flameretardant (c) added to the TPU to be from 0.1 to 60% by weight,preferably from 1 to 40% by weight, and particularly preferably from 5to 25% by weight, based on the total weight of the stabilized TPU.

In one preferred embodiment, the flame-retardant component (d) usedcomprises an amount of from 0.1 to 60% by weight, particularly from 5 to40% by weight, in particular from 15 to 25% by weight, of melaminecyanurate.

e) Suitable catalysts (e) which in particular accelerate the reactionbetween the NCO groups in the diisocyanates (a) and the hydroxy groupsin components (b) and (c) of the composition are the tertiary aminesknown and conventional in the prior art, e.g. triethylamine, dimethylcyclohexylamine, N-methylmorpholine, N,N′-dimethylpiperazine,2-(dimethylaminoethoxy)ethanol, diazabicyclo[2.2.2] octane, and thelike, and also in particular organometallic compounds, such as titanicesters, iron compounds, e.g. iron (III) acetylacetonate, tin compounds,e.g. tin diacetate, tin dioctoate, tin dilaurate, or the dialkyltinsalts of aliphatic carboxylic acids, for example dibutyltin diacetate,dibutyltin dilaurate, or the like. The amounts usually used of thecatalysts are from 0.0001 to 0.1 parts by weight per 100 parts by weightof component (b).

f) Where appropriate, auxiliaries and/or additives (f) may be added toprepare the polyurethanes of the invention. These are well known fromthe prior art. Mention may be made by way of example of lubricants,inhibitors, stabilizers with respect to hydrolysis, light, heat, ordiscoloration, dyes, pigments, inorganic and/or organic fillers, andreinforcing agents.

These auxiliaries and/or additives may be introduced into the componentsof the composition or into the reaction mixture for preparing the TPUs.In another version of the process, these auxiliaries and/or additives(f) may be mixed with the TPU and then melted, or are incorporateddirectly into the melt.

Besides the components mentioned a), b), and c), and, where appropriate,d) to f), use may also be made of chain regulators, usually with amolecular weight of from 31 to 499. These chain regulators are compoundswhich have only one functional group reactive toward isocyanates, e.g.monofunctional alcohols, monofunctional amines, and/or monofunctionalpolyols. In particular for TPUs, flow behavior can be adjusted in acontrolled manner via these chain regulators. The amount which maygenerally be used of chain regulators is from 0 to 5 parts by weight,preferably from 0.1 to 1 part by weight, based on 100 parts by weight ofcomponent b), and the chain regulators are defined as part of componentc).

The TPUs of the invention are prepared by reacting

a) polyisocyanates with

b) compounds having at least two hydrogen atoms reactive towardisocyanate, and

c) chain extenders,

d) flame retardant,

e) catalysts where appropriate, and

f) additives where appropriate,

the amount of flame retardant (d) used being such as to achieve the V-2,V-1, or V-0 classification in the Underwriters Laboratories UL 94vertical burning test, and the content of compounds having aromatichydrocarbon groups in components b) to f) being below 5% by weight,based on the total weight of the thermoplastic polyurethanes.

The reaction may proceed with the usual indices, preferably with anindex of from 60 to 120, particularly preferably with an index of from80 to 110. The index is defined via the ratio of the total number ofisocyanate groups used during the reaction in component (a) to thegroups reactive toward isocyanates, i.e. the active hydrogen atoms, incomponents (b) and (c). If the index is 100, there is one activehydrogen atom, i.e. one function reactive toward isocyanates, incomponents (b) and (c) for each isocyanate group in component (a). Ifthe index is above 100, there are more isocyanate groups present than OHgroups.

The TPUs may be prepared by the known processes continuously, forexample using reactive extruders or the belt process by the one-shotmethod or the prepolymer method, or batchwise by the known prepolymerprocess. In these processes, the components (a), (b), (c), and, whereappropriate, (d) to (f) being reacted may be mixed with one another insuccession or simultaneously, whereupon the reaction begins immediately.

In the extruder process, the components (a), (b), (c), and also, whereappropriate, (d) to (f) of the composition are introduced separately oras a mixture into the extruder, e.g. at from 100 to 280° C., preferablyfrom 140 to 250° C., and reacted. The resultant TPU is extruded, cooled,and pelletized.

After the synthesis, the TPU may, where appropriate, be modified byprocessing in an extruder. The melt index of the TPU, for example, orits pellet form may be modified in accordance with requirements via thisprocessing.

Components (d) to (f) may be fed during the synthesis or processing ofthe TPU. It is also possible to prepare concentrates which comprisecomponents (d) to (f) and to feed these into the TPU during processing.

The additives are preferably added in a compounding extruder, preferablyusing the two-screw process, where the TPU is fed in pellet form, thenmelted, and the addition of the additives takes place as the extrusionprocess proceeds. The melt is extruded, cooled, and then, in acontinuous process, pelletized, or else subjected to cutting directlyafter discharge from the die by way of an underwater pelletizer orwater-cooled die face pelletizer. However, where appropriate, it is alsopossible for the flame retardants (d) to be added before synthesis onthe belt plant or in the reactive extruder is complete.

The thermoplastic polyurethanes of the invention generate little smokeduring combustion. There are various methods available for testing thesmoke generated during combustion.

The smoke density may be tested in an NBS smoke density chamber to ASTME662-79. The attenuation of a light beam due to smoke collecting in thetest chamber is measured. The smoke is generated during pyrolysis of thetest specimen. The result is expressed in terms of a specific opticaldensity.

DIN 53436/53437 is also commonly used. Here, the plastic to be tested isdecomposed thermally in a quartz tube by means of an annular furnace,and smoke density is measured in the measurement device to DIN 53437.

Determination of the Indice de fumée to NF-F16-101 using the smokedensity curve to NF X 10-702 is a frequently demanded test, because theassessment here can be made with regard to both maximum smoke densityand smoke toxicity. Determination of the smoke density curve involves,inter alia, data on the maximum smoke density (Dm) and the degree ofsmoke-darkening after 4 minutes (VOF-4), these permitting an assessmentof fume generation.

The thermoplastic polyurethanes of the invention generate only littlesmoke during combustion. The maximum smoke density (Dm) generated duringdetermination of the Indice de fumée to NF-F16-101 using the smokedensity curve to NF X 10-702 is preferably less than 300, morepreferably less than 250, still more preferably less than 200,particularly preferably less than 150, and in particular less than 110.In addition, the degree of smoke-darkening after 4 minutes (VOF-4)apparent during combustion of the TPUs of the invention duringdetermination of the Indice de fumée to NF-F16-101, using the smokedensity curve to NF X 10-702, is preferably less than 650, morepreferably less than 450, still more preferably less than 300,particularly preferably less than 200, and in particular less than 150.

Conventional processes, e.g. injection molding or extrusion, are used toprocess the TPUs of the invention, which are usually in the form ofpellets or powder, to give injection-molded or extruded items, e.g. togive films, moldings, rollers, fibers, panels in automobiles, hoses,cable plugs, bellows, drag cables, cable sheathing, gaskets, belts, orattenuating elements. These injection-molded or extruded items may alsobe composed of compounds comprising the TPU of the invention, and becomposed of at least one other thermoplastic, in particular of apolyolefin, polyester, polyether, polystyrene, styrene copolymer, orpolyoxymethylene.

The thermoplastic polyurethanes of the invention and the moldingsdescribed above comprising the TPUs of the invention may be used in manyways, for example in means of transport, in electrical items, or inmachines. Examples of suitable means of transport are motor vehicles,such as cars or trucks, rail vehicles, aircraft, and ships. Examples ofelectrical items are household devices, televisions, stereo systems,video recorders, computers and accessories, printers and accessories,copiers and accessories, scanners and accessories, switchgear cabinets,and control systems. Examples of machines are packaging machines.,robots, wood- or metal-working machines, machine tools,injection-molding machines, extruders, calenders, blown-film machines,CAD machines, milling machines, stamping machines, presses, turningmachines, construction-site machines, e.g. excavators, wheel loaders,cranes, conveying systems, industrial trucks, sorting machines, conveyorbelts, process monitoring systems, and process control stations.

The invention will be illustrated by the following examples.

EXAMPLES Example 1

A TPU was prepared in the laboratory by the one-shot process. Use wasmade of 1.0 mol of PTHF 1000 (polytetrahydrofuran with molecular weight1 000), 2.4 mol of 4,4′-diphenylmethane diisocyanate and 1.4 mol of1,4-butanediol as chain extender. The PTHF was preheated to 70° C., andthe 1,4-butanediol was added, and the diisocyanate was incorporated bymixing at 65° C. Once the temperature had reached 110° C., the mixturewas poured into a Teflon mold, onto a heated bed, and after 10 minutesplaced in a heating cabinet and heat-conditioned at 80° C. for 15 hours.After the heat-conditioning, the material was ground and dried, andinjection-molded plaques were produced.

Example 2

A TPU was prepared in the laboratory by the one-shot process. Use wasmade of 1.0 mol of PTHF 1000, 2.4 mol of hexamethylene diisocyanate and1.4 mol of 1,4-butanediol as chain extender. The PTHF was preheated to80° C., and the 1,4-butanediol was added, and the diisocyanate wasincorporated by mixing at 75° C. Stannous octoate was used as catalyst.Once the temperature had reached 110° C., the mixture was poured into aTeflon mold, onto a heated bed, and after 10 minutes placed in a heatingcabinet and heat-conditioned at 80° C. for 15 hours. After theheat-conditioning, the material was ground and dried, andinjection-molded plaques were produced.

Examples 3, 4, and 6

The synthesis takes place as stated in example 1. The amount stated intable 1 of flame retardant was added to the polyetherol PTHF and heatedto the starting temperature.

Example 5

The synthesis takes place as stated in example 2. The amount stated intable 1 of flame retardant was added to the polyetherol PTHF and heatedto the starting temperature.

The specimens obtained in examples 1 to 6 were subjected to a smokedensity test to NF-F16-101 (Index de Fumée). The results are listed intable 1. All data in table 1 relate to parts by weight. In the case ofthe UL 94 V combustion test “-” means not passed. TABLE 1 Ex. 1 Ex. 2Ex. 3 Ex. 4 Ex. 5 Ex. 6 Aromatic TPU 100    80 80 90 Aliphatic TPU 10090 Diphenyl cresyl-    20 phosphate Tributyl phosphate 20 10 10 Dm 97 70   685 260 100 200 maximum smoke density VOF-4 190 20 1 360 605 125 420degree of smoke darkening after 4 min UL 94 combustion test — — V-2 V-2V-2 V-2

Examples 1 and 2 show the comparison between aromatic TPU and aliphaticTPU without addition of any flame retardant. In these two examples,although the desired combustion with little generation of smoke isachieved the desired flame retardancy is not achieved (combustion testto UL 94 V not passed).

Examples 3 and 4 show the comparison between aromatic and aliphaticflame retardant in TPU based on aromatic isocyanates. Example 3 liesoutside the claimed range of the invention and does not provide thedesired combustion with little smoke generation.

Examples 5 and 6 show the comparison between a TPU based on an aliphaticor aromatic isocyanate when use is-made of an aliphatic flame retardant.It can be seen that the use of aliphatic flame retardants and ofaliphatic isocyanates is particularly preferable.

1. A flame-retardant thermoplastic polyurethane prepared by reacting a)one or more aliphatic polyisocyanates with b) one or more compoundshaving at least two hydrogen atoms reactive toward isocyanate, and c)one or more chain extenders, d) one or more flame retardants, e) one ormore catalysts where appropriate, and f) one or more additives whereappropriate, the amount of said one or more flame retardants (d) usedbeing such as to achieve the V-2, V-1, or V-0 classification in theUnderwriters Laboratories UL 94 vertical burning test, and the contentof compounds having aromatic hydrocarbon groups in components b) to f)being below 5% by weight, based on the total weight of the thermoplasticpolyurethane.
 2. The thermoplastic polyurethane as claimed in claim 1,wherein the amount of the flame retardant d) is from 5 to 50% by weight,based on the total weight of the thermoplastic polyurethane.
 3. Thethermoplastic polyurethane as claimed in claim 1, wherein the component(b) comprises a polyesterol comprising an alkylene glycol adipate.
 4. Aprocess for preparing flame-retardant thermoplastic polyurethanes byreacting a) one or more aliphatic polyisocyanates with b) one or morecompounds having at least two hydrogen atoms reactive toward isocyanate,and c) one or more chain extenders, d) one or more flame retardants, e)one or more catalysts where appropriate, and f) one or more additiveswhere appropriate, the amount of said one or more flame retardants (d)used being such as to achieve the V-2, V-1, or V-0 classification in theUnderwriters Laboratories UL 94 vertical burning test, and the contentof compounds having aromatic hydrocarbon groups in components b) to f)being below 5% by weight, based on the total weight of the thermoplasticpolyurethanes. 5-7. (canceled)
 8. The thermoplastic polyurethane asclaimed in claim 1, wherein said thermoplastic polyurethane, inaccordance with the Indice de fumée of NF-F16-101, have a maximum smokedensity (Dm) of less than 300, and have a maximum degree ofsmoke-darkening after 4 minutes (VOF-4) of less than
 650. 9. A film,molding, roller, fiber, panel in an automobile, hose, cable plug,bellow, drag cable, cable sheathing, gasket, belt, or attenuatingelement, wherein said film, molding, roller, fiber, panel in anautomobile, hose, cable plug, bellow, drag cable, cable sheathing,gasket, belt, or attenuating element comprises said thermoplasticpolyurethane as claimed in claim 1.